Organic electroluminescence devices are devices in which when charges are injected into an organic light emitting layer disposed between an electron injecting electrode (cathode) and a hole injecting electrode (anode), electrons and holes combine with each other in the light emitting layer and then the electron-hole pairs decay to emit light. Organic electroluminescence devices can be fabricated even on flexible transparent substrates, such as plastic substrates. Other advantages of organic electroluminescence devices are low driving voltages of 10 V or less, relatively low power consumption, and accurate color representation compared to plasma display panels and inorganic electroluminescence displays. In addition, organic electroluminescence devices can represent green, blue, and red colors. Due to these advantages, organic electroluminescence devices have been the subject of intense interest as next-generation full-color display devices.
Luminescent materials are the most important factors determining the luminous efficiency of organic electroluminescence devices. Fluorescent materials are widely used at present as luminescent materials but the development of phosphorescent materials is theoretically considered an approach to further improve the luminous efficiency of organic electroluminescence devices in view of the mechanism of light emission. Thus, various phosphorescent materials have been developed and are currently being developed. Particularly, 4,4′-N,N′-dicarbazolebiphenyl (CBP) is most widely known as a phosphorescent host material. Organic electroluminescence devices are known that use, as hosts, carbazole compounds whose carbazole skeletons are substituted with various groups (Japanese Patent Publication Nos. 2008-214244 and 2003-133075) or BALq derivatives.
Organic electroluminescence devices using phosphorescent materials have considerably high current efficiency compared to devices using fluorescent materials. However, organic electroluminescence devices using BAlq and CBP as phosphorescent host materials do not offer significant advantages in terms of power efficiency over devices using fluorescent materials because of their higher driving voltages and do not reach a satisfactory level in terms of device life. Under these circumstances, there is a need to develop a more stable high-performance host material.